ABSTRACT
Cells are the key building blocks of living systems. Some are self-sufficient while others function as parts of multicellular organisms. Biomembranes compartmentalize areas of different metabolic activities within cells. They also regulate the flow into and out of cells and cell compartments. A cell membrane is the thin nearly invisible structure that surrounds the cytoplasm. A large amount of diffusion in biological organisms takes place through membranes. The function of a cell membrane generally revolves around its proteins. The cell membrane can also engulf structures too large to fit through the pores in the membrane proteins. If the cell is stimulated by mechanical, chemical, or electrical means, sodium ions diffuse more readily into the cell since the stimulus changes membrane permeability. A cell membrane has pores or channels that allow selective passages of metabolites and ions in and out of the cell and can even drag molecules from areas of low concentration to high concentration working directly against diffusion.
